AMD has a bug with 2x2 and 1x1 mips of depth/stencil textures. Formats
D24S8 and D32FS8 both seem to render black for the very small mips.

We can work around this by selectively caching a copy of the texture
data that only has depth information, since stencil isn't filterable
in OpenGL ES.

BUG=angleproject:1493
BUG=chromium:638323

Change-Id: Iebef42c4680018d9854dbe789d677823167213dd
Reviewed-on: https://chromium-review.googlesource.com/380037Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>